An antenna for transmitting and receiving an electromagnetic wave using a magnetic material (hereinafter referred to merely as a “magnetic antenna”) in which a magnetic field component coming from outside is allowed to pass through a core (magnetic material), around which a coil of a conductive wire is wound, to convert the magnetic field component into a voltage (or current) induced by the coil, has been widely used in small sized radios and TVs. Such a magnetic antenna is also used in a non-contact object identification device called an RF tag which has recently widely come into use.
To transmit and receive an electromagnetic wave with a higher frequency, a planar loop coil which is free of a magnetic material and has a coil surface parallel to an object to be identified is used as an antenna in RF tags. When the frequency is much higher (UHF band or microwave band), an electric field antenna (dipole antenna or dielectric antenna) for detecting an electric field component instead of a magnetic field component is widely used in such RF tags.
However, the planar loop antenna and electric field antenna have the following problems. That is, when such an antenna comes close to a metallic object, an image (mirror effect) is generated on the metallic object. Since the image has a phase opposite to that of the antenna, the sensitivity of the antenna tends to be lost.
On the other hand, there is also known a magnetic antenna for transmitting and receiving a magnetic field component which comprises a magnetic layer as a central core, an coil-shaped electrode material wound on the core, an insulating layer formed on one or both outside surfaces of the core on which the coil-shaped electrode material is provided, and a conductive layer formed on one or both outside surfaces of the insulating layer (Patent Document 1). The magnetic antenna described in Patent Document 1 can maintain properties required for antennas even when coming into contact with metal articles. There are also known tags or magnetic antennas which are installed under specific conditions (Patent Document 2).
Further, it is conventionally known that, in RF tags for transmitting and receiving information using an electromagnetic induction method, surrounding water, if any, has adverse influence thereon, i.e., causes increase in dielectric constant around the RF tags, resulting in deviation of resonance frequency of the RF tags and therefore deterioration in communication characteristics thereof.